This invention relates to impellers for centrifugal pumps, and more specifically to a process for manufacturing impellers.
A pump impeller is composed essentially of three parts: a hub portion, vanes, and a shroud portion. Traditionally, there have been several ways to manufacture impellers. In one method, molds for the vanes are placed on top of a hub. The molds are arranged as desired for the vanes, and filled with weld material. The material fills the mold, forms the vanes, and is attached to the hub. The shroud must then be attached to the vanes.
Several other manufacturing methods use pre-fabricated vanes, which are then welded in place between the hub and the shroud. In another method, a set of long vanes are attached to the hub, extending from the edge to the center. A set of shorter blades which do not extend all the way to the center of the hub are attached to the shroud. The complementary hub and shroud are then attached, with the vanes interleaved.
Other manufacturing methods deform the impeller pieces into their respective shapes, and then attach them to each other. The impeller parts may also be cast, and then assembled.
Another impeller manufacturing method creates two pre-formed parts--a hub, having the vanes attached to it, and a shroud. These pieces are placed in appropriate position relative to one another by means of a five-axis machine. Once in proper position, the pieces are then electron-beam welded together. Both the use of a five-axis machine, and the process of electron-beam welding are quite expensive and require long lead times for production. Further, this method of fabricating impellers allows the potential for crevice erosion, since electron beam welding does not make a complete joint from one side of each vane to the other. There is also the potential for poor mating between the hub-vanes portion of the impeller and the shroud portion of the impeller, since the respective surfaces of the shroud and vanes that are in contact with each other may not match up, and the intersection of the surfaces may leave gaps between them. In addition to allowing voids or gaps between the surfaces, when the shroud and the vanes are not specifically matched to one another, there is a likelihood of improper or imprecise alignment. In addition, the integrity of electron-beam welds between the shroud portion and the hub-vanes portion is unknown, in part due to the variation in surface contact. Finally, the cost of machining, assembling and welding the shroud and hub-vanes portions of the impeller, first utilizing a five-axis machine and then subsequently electron-beam welding the portions together is very high, both from a monetary and a time-management viewpoint.
Thus there exists the need for an economical process to manufacture high-strength, precision impellers, including hub, vane, and shroud components which fit together in a mating fashion, preventing large gaps between the components and ensuring proper alignment of the components.